Mr Shatter said he hopes today’s statement “represents progress for the Stack family in their quest for the full truth behind the brutal killing of Brian Stack”.

However he added: “the stark reality is that the Provisional IRA not only carried out this brutal killing but have hidden their involvement in it for decades.

Mr Shatter continued: “It is also a matter of great regret that in an admission of responsibility which comes 30 years too late attempts have been made to put this killing in the context of an allegedly brutal regime at Portlaoise Prison.”

Brian Stack was gunned down outside the National Stadium in Dublin in 1983, and died 18 months later.

His family have been fighting for the truth about his death for three decades, with Sinn Fein leader Gerry Adams finally agreeing to meet Mr Stack’s sons Austin and Oliver in May.

That meeting led to the pair being driven in a blacked-out van last week to an undisclosed location where a former IRA chief admitted responsibility.

The IRA statement read: "This action was not authorised by the IRA leadership and for this reason the IRA denied any involvement".

"Some years later, when the Army Council discovered that its volunteers had shot Prison Officer Brian Stack, the volunteer responsible for the instruction was disciplined."

But the statement said the IRA killers involved were acting under orders.

"This operation should not have taken place," it states.

Mr Adams said he wanted to pay tribute to Austin and Oliver, their brother Kieran and mother Sheila.

"On behalf of Sinn Fein I extend my regret at the killing of Brian," he said.

He said the meeting has brought an element of closure for the family but they still have many unanswered questions, including the identity of the killers and why the Garda probe was "compromised".

Speaking of the Garda investigation, Mr Shatter said the case has been the subject of a review by the Serious Crime Review Team and is with the National Bureau of Crime Investigation.

He said: “While I have been aware of the frustration of members of the family at the difficulties which have arisen in establishing the truth of this case, we should not underestimate the challenges faced by An Garda Síochána when persons will provide information to others but not cooperate with their inquiries.

Mr Shatter appealed for new information on Mr Stack’s killing saying: “It should be clear to everyone that what is needed now is for anyone who has information about this case to face up to their responsibilities and cooperate fully with An Garda Síochána in their investigation, rather than hide behind platitudes about difficulties in addressing the past.”

“It is a simple and tragic fact that the killing of Brian Stack was a callous and premeditated murder which cannot be justified or explained away.”

Mr Shatter said: “Chief Officer Stack was not only a dedicated and committed servant of the Irish State but a loyal husband and father.

“His murder was not only a tragedy for his family but an attack on the foundations of this State.”